We originally cloned SAFB1 as a transcriptional repressor, and it has since been implicated in a number of cellular processes including stress response, apoptosis, RNA processing, hormone response, immortalization, and transformation. During the last funding period we performed detailed structure function studies which identified a transcriptional repression domain, we found that sumoylation of SAFB1 was necessary for its co-repressor activity, and finally, we generated and characterized SAFB1-null mice which showed prenatal and neonatal lethality with surviving mice showing dramatic defects in the development and function of the reproductive system. To gain more insight into SAFB1's role in ERalpha action, we performed an unbiased screen using SAFB1 siRNA knockdown followed by microarray analysis. This experiment revealed a significant role for SAFB1 in estrogen-mediated repression of gene expression, and candidate genes that require SAFB1 for estrogen-mediated downregulation were identified. Here we propose to study how SAFB1 mediates estrogen repression of gene expression. Specifically, we will i) determine how SAFB1 SAFB1 mediates transcriptional repression of estrogen-responsive target genes, ii) characterize HDAC7 as a critical player in SAFB1-mediated repression of estrogen regulated genes, and iii) determine how posttranslational modification by sumoylation affects SAFB1's co-repressor function. This work is highly significant since it provides studies of fundamental mechanisms which may contribute to a change in paradigm of ERalpha's action. While ERalpha has mainly been studied as a transcriptional activator, the study of estrogen- mediated repression of gene expression, and in particular the role of co-repressors, is an understudied area. 1